Excerpt from "Evolution of Naval Radio-Electronics and Contributions of the Naval Research
Laboratory" - Louis A, Gebhard, GPO 1979.

VLF Transmission

The transition from the international Morse code keying transmission system to
the automatic teleprinter system came about much later on the very-low-frequency
circuits than on those in the high-frequency band, This was primarily due to the extremely low bandwidth characteristic of the huge shore station antennas at the very-low
frequencies, which imposed a limit on the speed of transmission (20 words per minute,
continuous wave, international Morse code keying) to about one-half that required for
teleprinter operation at its lowest speed. In overcoming this limitation, NRL devised
the first teleprinter system providing effective operation on the very-low-frequencies
(1951) . The performance of this system was demonstrated in operations using
transmissions from the Navy's VLF station at Annapolis, Maryland (NSS, 15.5 kHz) over long
distance circuits to Iceland, England, Panama, Canal Zone, and North Africa. This system
was self synchronizing and provided the encoding of a standard teleprinter
signal into a four level signal having one half the keying rate of the original, The transmitter
was shifted through the four frequency levels by the encoded signal which, as modified, could then be
accommodated by the bandwidth of the antenna, At the receiver, a decoding device converted the received four level signal back into its original
form for operation of the teleprinter. A novel, stable, regenerative circuit
provided a much higher degree of selectivity in the frequency-shift receiver than had previously
been attained (25 Hz bandwidth), A specially designed discriminator permitted segregation
of the signals on the four frequency levels, which were separated by a very small difference in frequency (4 Hz).

With the advent of the Polaris weapon system, grave concern arose
regarding the reliability of command and control communications via the Navy's VLF
transmitting system. In responding to this situation, NRL developed a VLF facsimile transmission
system which was first to provide reliable command and control communication from a
single high power transmitting station in the United States to continuously submerged
submarines when operating in any critical world area (1959). Early in 1959, the submarine
USS KATE used the system successfully on its trip to the North Pole, The
submarine USS TRITON, in accomplishing the first circumnavigation of the globe, submerged,
used the system throughout the voyage with good results (February-May 1960). The system
was installed on all Polaris submarines and provided highly reliable command-control
communications during the critical period that followed, This system became known
as "Bedrock". The Navy's existing transmitting system had to contend with high atmospheric
noise levels prevalent at the very low frequencies which produced low signal to noise ratios and seriously affected the reliability of communications in distant
areas of operational importance, such as the Mediterranean Sea, In the system devised,
the superior performance obtained under extremely low signal to noise ratio conditions
was achieved through the use of very narrow frequency bandwidth transmissions and the
redundancy provided by facsimile type signaling, A facsimile-controlled exciter provided the small frequency shifting of the
transmitter. The frequency-shift receiver utilized the novel techniques for high selectivity and discrimination previously devised
for the VLF teleprinter system, Transmitter components were provided for installation
at shore stations: NSS, Annapolis, Maryland; NAA, Cutler, Maine; NPM, Lualualei, Hawaii;
NPG, Jim Creek, Washington; and NBA, Summit, Panama Canal Zone (1958-1964). Receiver
components were supplied for submarines, the first installation being made on the USS
SABLEFISH (January 1959) . In the trials of the system made with this submarine in the
Mediterranean Sea area, excellent submerged reception results were obtained on transmissions
from the station at Annapolis, Maryland. Similar results were obtained by the submarine
USS BANG at its station in the North Atlantic off Norway (February 1959).

NRL developed a frequency-shift keyer which for the first time permitted automatic operation of the Navy's VLF transmitters at a rate as high as 60 words per minute with a high degree of reliability for command-control communications to Polaris submarines (1963) . All of the Navy's high power stations were then equipped with these keyers.

The system utilized two frequency levels for keying with provision to avoid the large voltage and current transients previously experienced when the large quantity of oscillatory energy in the antenna system was abruptly changed in frequency. These transients had, at times, caused a flashover of "horn-gaps" and other protective devices followed by objectionable shutdown of transmitters due to overload. In certain instances, critical damage occurred, such as the burnout of antenna loading inductance cable, rendering the station inoperative for a considerable period. The transients were avoided by beginning each successive "mark" and "space" frequency shirt at the zero-crossing points of the "mark" and "space" frequencies, when these points were coincident in phase, and arranging the rate of change of frequency to be linear during the transition process. The transition period was of such length as to hold the sideband energy generated during transition within the frequency bandwidth of the antenna, Maximum utilization of the antenna bandwidth was obtained by very precisely maintaining the "mark"
and "space" frequencies; this was possible with NRL-devised techniques. Full utilization of the antenna bandwidth and confinement of the sideband energy to within its limits are major factors in maximizing the rate of transmission,

Info from John Cobb:
"The AN/FRT-10 was unique, a 500 kW LF giant tuning 100 - 200 kHz. It was in two amplifier bays, which we drove alternately with an external TMC
synthesized exciter stack, running the single-channel KSUB broadcast at about 50 kW. It fed a 1200-foot tower, through 8-inch rigid coax and a big
variometer/capacitor network in the helix house. The amplifier bays each
consisted of IPA and driver stages for the finals: four air-cooled 6697 triodes in grounded-grid push-pull configuration!"

Info from Roy K1LKY "I was fortunate to visit the VLF station at NSS Annapolis before it was dismantled. On that trip we learned that the narrow shift FSK keying put the carrier alternately on either side of the center of the tuned system. It was some 10 percent down the slope (memory of the details are faint.) But the antenna current was monitored at the console and would vary depending on how far down the slope the tuning had shifted the center of the very narrow sweet spot. The operator could vary one of the inductors in the system by remote control to match the two currents. That system operated at 20.4 kc I believe, at an output power of about eight tenths of a megawatt. The shift may have been on the order of 10 to 15 CPS. The antenna was one mile long, about 800 to 1200 feet up, and was tuned with a massive inductor made of five inch diameter
Litz wire. Cutler Maine still operates with similar parameters, as far as I know."

TACAMO radio equipment was manuf by Collins - please send
e-mail if you have any more info

EC-130G is EC-130E with TACAMO communication system AN/USC-13(V)
installed.
I think the EA-6A had the same system
The 200KW tube-type transmitter is OG-193T/USC-13(V) Amplifier Coupler
Group.

The VLF set in the E-6B is OZ-1/USC-13(V) and the 200KW solid state transmitter is
the AN/ART-54 HPTS
Solid State Power Amplifier/Coupler (SSPA/C)
OG-187/ART-54
Dual Trailing Wire Antenna System (DTWA) OE-456/ART-54.

The TACAMO mission began in 1961 as
a test program to determine if an airborne Very Low Frequency (VLF)
communications system was feasible. Weapons Systems Test Division
conducted this program, using a U.S. Marine Corps KC-130 aircraft as a
test vehicle. The overwhelming success of the test program
prompted funding for the first production aircraft. The project
was designated TACAMO, which stands for “Take Charge and Move Out.”

In order to avoid long lead-time delays, four U. S. Air Force
C-130 aircraft were taken from the production line in early 1963.
After extensive modification, the aircraft was designated a C-130G.
The first of these aircraft, BUNO 151890, was delivered on 26 December
1963. At that time, the communications equipment consisted of
removable vans that could be installed in a matter of hours. Plans
were formulated in 1966 to expand the TACAMO Program. The
expansion included permanently installing the communications suites in
eight aircraft and their designation as EC-130Qs. On 1 July 1968,
VQ-4 was established at NAS Patuxent River, MD as a permanent
operational squadron.
In 1974, the next major aircraft modification incorporated a new
power amplifier, a dual trailing wire antenna system and a high-speed
reel system for deploying and retracting of the trailing wire antennas.
Additional improvements included satellite communications and an
enhanced VLF capability.

In the 1980s, it was recognized that the C-130s, in some cases the
oldest in the fleet, were in need of replacement. The hunt
was on for the successor to “The Mighty Herc.” Eventually, the
B707-320 airframe was chosen, modified extensively and designated the
E-6A. The Navy E-6s were the last 16 aircraft to roll off of
Boeing’s venerable 707 line after 30 years of production.
On 25 January 1991, VQ-4 took delivery of its first E-6A Mercury
aircraft and in November 1992, changed homeport to Tinker AFB, Oklahoma
City, Oklahoma. On 20 September 1999, VQ-4 took delivery of its
first E-6B. The E-6B contains upgraded systems that enable it to
perform the USSTRATCOM Airborne Command Post (ABNCP) “Looking Glass”
mission. Upgrades include: a Battle staff module to provide
enhanced command, control and communications for the Nation’s nuclear
arsenal; the Airborne Launch Control System to permit airborne launch
and control of ICBMs; UHF C3 Radio Subsystem; Digital Airborne
Intercommunications Switching System; MILSTAR Airborne Terminal System
(Satellite communications), and a High Power Transmit Set for enhanced
communications.